I am trying to install Exchange Server 2013 for days. On every try I am getting the error "MapiExceptionUnknownUser: Unable to open message store." while connecting to either ECP or OWA.

In my searches, some people says it is about Reginal Settings. I made a clean install for both Windows Server 2012 and Exchange Server 2013. I have set all reginal settings to English and US. I have installed a server for Active Directory and a seperate server for Exchange. I made everything decent and there was no error at prerequisites. There was no error after installation. But every time I try to reach ECP or OWA I get error about user.

Somebody told me creating a user on AD without mailbox would fix the problem but no.

I could open ECP page by https://< IP >/ecp/?ExchClientVer=15 with ?ExchClientVer=15 This URL doesn't work for OWA.

The problem is about using some language other than English (US) for operating system. If you are using any other language including English UK, Exchange may crash on setup. So, you may start with a clear OS setup or try changing display language and also Regional settings to US definitions.

You can only use keyboard with some language other than English US. Anything else have to be English US. Your display language, regional setting, time etc.

My old answer here:
Here is the solution. It is explained with photos.